Method and apparatus for sound analysis



Feb. 1, 1944.

A. E. TRAvr-:R Erm.

v METHOD AND APPARATUS FOR SOUND ANALYSIS Filed Jan.l 4,- 1941 /m/ocfr Mmm/MEAN P/w/vc/Pzf DETECTOR OUTPUT 2 Sheets-Sheet 2 METER @E40/NG 14V). l l

90 so' 4o J0 Aff/.55 PER How? new f. EASE@ Ficha narran stares Parati-'r cerise METHOD AND APPARATUS FOR SOUND ANALYSIS Ali'red E. Travel', Brooklyn, and Carl E. Habermann, Great Neck, N. Y., asslgnors to Socony- Vacuum Oil Company, Incorporated, New York,

N. Y., a corporation ci! New York Application January 4, 1ML-Serial No; 373,070v

(Cl. 'I3- 51) 4 Claims.

tive knock intensity of various fuels by ear or other device. In general, these devices may be divided into two classcations; rst, those which aid the ear to determine the ioudness of the knocking conditions and to determine the relasound. and second, those-which indicate the loudness of the sound on electrical meters without use of the human ear. c,

The human ear varies in sensltivity'to sound for different individuals and the keenness o1 hearing of one individual will varyV constantly throughout the day, since the organs which are used in hearing tend to become tired with use and their sensitivity temporarily lowered. Therefore, any device or method for determining Y. the loudness or detonation which uses the human ear is unsatisiactory. y l

lThe devices which have been built to indicate the loudness oi detonation on meters consist es sentally of two types; rst, the sound level meter.- and second, the sound analyser. These instruments cannot'be used successfully for the followin reason: Y

The soundof detonation occurs for a very short interval of time and usually reoccurs at constant intervals. The time of duration of the loud soundv lof knock may only be one percent or less of the duration of time between the reoccur'rng detonation sounds. Therefore, it is very dithcult to de ,termine the sound intensity on metersvwhich tend to measure the average sound intensity throughout the engine cycle.

The object of this invention is to provide a detonation meter incapable of reecting variations in mechanical setups and operator respouses. A further object is the provision cf ra meter capable of being adjusted forreception and measurement of knock, accompanied bythe suppression of extraneous engine noise.V

A11 of these objects, as well as. others which are hereinafter indicated, flowfrom the particular character of this novel meter. where characteristics may be understood by reference to the drawings attached hereto. In these drawings Figures 1 and 2 vshows in schematic form the wiring setup of the meter. Figure 3 shows in principle the method of knock measurement, and Figure t shows a sample result sheet arising from the use of the meter. Y I

The knock comparatorconsists of the ioliowing essential parts. A microphone. vacuum tube amplifiers, filters, vacuum tube detector, and an indicating meter. All of the parts, with the exception of the microphone, are built into 'afportable metal case which also contains the batteries necessary for its operation.

- 'Thersound of the knock and other noises'are pickedup by a microphoneFigure 2, item 5, sensitive to sound frequencies from 40 to 10,000 cycles per second. The microphone converts the sounds to equivalent electrical waves. The electrical waves are passed through an attenuator 6, and through three amplifying stages, l, 8., 9, be-j `fore being impressedon the grid of the Vvacuum tube detector, l0.- There is' a band pass filter Il, interposed' between the first and second vacuum-tube amplifier stages and asimilar filter is preferably inserted'between the second. and third stages. The filters are ofthe tuned inductancecapacity type and the coupling between vthe ampliers is of the conventional resistance-capacity The filters are not sharply tuned, for it was desired to obtain a broad band of soundfrequcncy which would pass the'filters. -In case of .opcratioiri with a single type of motor, the filters maybe xed at a preselected level, say approximately 4000 cycles; however, for most uses it is desirable to use variable frequency iilters. inordel'. that the meter may be adjusted for use with variousA motors in which knocking occurs at .various frequencies, ranging usually between 2090 and 6000 cycles per second. The frequency of 'the filter to beused may be determined by experiment and will vary with dierentautomoblles. The

e filter should be set to pass a frequency band which will give the best discrimination between the sound of the knock and of other engine noises.

The detector circuit consists of a coupling con.. denser l2 and a vacuum tube' grid leal; i3 cf such valueslof capactyand resistance that the effect of an electrical wave of highintensity will be maintained for a period of Vtime many times longer than its own duration. In this application it is desirable to make the time constant of the condenser-resistor combination somewhat longer than the expected interval between the successive detonations.

The effect of this method of handling the de which. im-,

tector tube may be seen by reference to Figure 3. In this figure, Irl-I4, is a time axis. Vertical lines i5, I5, I 6, IB, etc., indicate the occurrence of sounds along this time axis, and their length vertically indicates their relative strength. Lines I5, I5, indicate the strong sounds arising from knocking. Lines I6, I6, indicate other sounds arising from general motor noise. Remembering that the filter system has removed engine noises Vof frequencies other than those of the sounds of knocking, it will be realized that those few engine noises of frequencies such that they pass the filter are relatively quite unlikely to be of intensity or strength comparable to the sound of knocking. In electrical terms, the height of any line I5 (or IS) is the voltage impressed upon the condenser I2 in the detector circuit. Thus that condenser is charged to successive levels I1, I1, etc., and the charge drains therefrom at a substantially constant rate determined by grid leakv I3. giving outputs of substantially constant rate of decrease, as shown by lines I3, in Figure 3. Now, if no sound I S is of sumcient intensity to rise above the level of line I8 at the instant of arrival of sound I6, then the extraneous sound will have onlya slight effect on the output of the detector. The meter reading, as indicated by line I9, is a further damped summation of the detector output. proportional thereto, but Without the momentary variations existing in detector output.

What this operation amounts to in simpler terms is this. First the microphone Aconverts all engine noise includingA knock into electrical impulses. Next we discard electrical impulses corresponding to sounds having frequencies other than the frequency observed for the sounds of knocking. Next we discard impulses corresponding to sounds having the same frequency as knock sounds and occurring in time intervals immediately following the knock sounds. What is left consists of a mixture of electrical frequencies of two amplitudes; a background of low amplitude impulses, corresponding to the miscellaneous small noises of the same frequency as knock sounds but not related thereto, which is designated as the engine noise level, and high amplitude impulses corresponding to the knock sounds, the average intensity of which may be measured and spoken of as engine knock.

The meter circuit has be'en designed so that a decrease in current through the detector tube. which will becaused by an increase in knock intensity, will be indicatedas an increase in meter reading. A manually controlled variable resistor 23 is used in the plate circuit of the detector tube so that the meter 24 may be set to zero at any level of sound intensity.

In actual service the microphone is placed in the engine compartment of an automobile by some suspension relatively incapable of mechanically transmitting engine vibration to the microphone, and is connected to the meter box by means of a shielded cable. The meter box is placed in a position convenient to the observer.

The automobile is operated in normal manner with a fuel non-knocking under normal conditions of operation, to determine the noise level incident upon such operation; which may be expressed, if desired by a curve such as curve 20 in Figure 4. If investigations of knocking at a single speed are to be made, only the noise level at that speed need be 'etermined, but inthe usual case, knocking overlmaegedlsration range is investigated. Then .the auto is cated under knocking conditions and the meter 2 sted to give center scale reading at the conditions of knock encountered. These operations, conducted together, and preferably in reverse order to that given, serve to adjust the meter 24 to the motor with which it is to be used in the test contemplated.

This invention is superior to thel prior art in several ways. First. it does not depend upon the human ear. Second, the meter readingr depends upon the loudest sound reaching the detector circuit and not upon the average sound level. Third, sounds which are not caused by the knock tend to be eliminated from the meter reading by electrical wave filters. Fourth, the meter 2# is well damped so that the eye can follow the indication of knock intensity. Fifth. the device is in a portable form and suitable for use on automobiles on the road.

We claim:

1. That method of investigating the knock' ing lcharacteristics of an internal combustion engine fuel, comprlslng burning the fuel in an engine under knocking conditions, observing all of the sounds emanating from said engine with a microphone capable of converting sound to electrical impulses, passing all of said impulses through an electrical filtering system to elimi-, nate therefrom sounds of frequencies different from the frequency of the knock sounds, passing the vremaining impulses through an electr-ical system having a time-lag feature acting to substantially eliminate impulses of the same frequency occurring at time intervals differing from the time interval of the knock sounds, and measuring the relative strength of the remaining impulses as a measure of the knocking in-v tensity of the fuel under examination.

2. That method of analyzing a sound complex including sounds of varying frequencies, a particula'r recurring sound which it is desired to investigate and other sounds recurring at intervals different from the particular recurring sound and which may have a frequency similar to the particular sound, comprising converting the sound complex to a complex of electrical impulses, filtering out impulses corresponding to sounds of frequencies dierent from the par- ,Y ticular sound, ilterngl out impulses of desired frequency but occurring at time intervals differing from the recurring particularsound, by generating in a ltriade an output current Whose value is substantially proportional to the intensity of each recurring particular sound and substantially constantly diminishing said output current. between each recurrence of said particular sound at a rate so selected as to prevent substantial changes in triode output current caused by sounds of like frequency and lesser intensity occurring between said recurring particular sounds, andobserving the value of said triode output current as a measure of the characteristics of the recurring particular sound.

3. In an apparatus for investigating the knocking characteristics of an internal combustion engine fuel, having means capable of converting sounds into electricall impulses for detecting all of the sounds emanating from said engine when said engine is in operation, means connected to said detectingA means for amplifying said electrical impulses, a filter connected to the output of said amplifying means adapted to pass sounds' of selected frequencies, additional amplifying means vconnected to the output of said filter means for amplifying the signals of selected frequencies, electronic detector means Hire,...

assegni c 3 for detecting said Aamplified signals, Vand means connected to the output of said detector means for indicating said detected signals, the improvement thatA comprises common means in the in-put circuit o f said detector means for' coupling said detector means to said amplifying means and for maintaining a wave of high.

intensity for a pre-determined period of time that is longer than its own duration, said means comprising a-condenser and grid l'eak, the values of which bear a ratio to each other such that the condenser will -havea -pre-determined time discharge constant in order that 'the duration of the-high intensity signal canbe maintained a controlled period of time.A l W Y 4. In an apparatusY for investigating the knocking characteristics of an internal combus-v tion engine fuel, having means capable of converting sounds into electrical impulses for detecting all of the sounds emanating from 'said engine when said engine is in operation;A means connected to said detecting means foramplifying said electrical impulses,a lter connected to 'the' output of 'sad amplifying means adapted to pass comprising a condenser and grid leak, the values sounds of selectedfrequencies; additional am- ,Y

plifying means connected to the output o! said filter means for amplifying the signals of selected frequencies, electronic detector means for detecting said amplified signals, and means connectedV to the out-put of said detector means for indicating said detected signals, the improvement that comprises common means in theinput circuit of. said detector meansrforrcoupling, l

said detector means to -said last amplifying means and-for maintaining ajgye 0i )Lightin- `tensity`for a pre-determined period of time that is longer than its own duration, said, means of which beara ratio to each other such that the condenser will have a pre-determinedV time discharge constant in order that the duration of the high intensity signal can be maintained a controlled period oftime, and means for adjusting said indicating means to eliminate from the detected values signals corresponding to undesired background motor noises.

ALFRED E. TRAVER. CARL E.. HABERMANN. 

